1. Field of the Invention
The present invention relates to a method of determining when the end of life of a battery is approaching, and circuitry which may be used with that process. The invention particularly relates to such methods where access to the battery for conventional testing means is not readily available, where maintenance of sufficient levels of power from the battery are important with minimal interruption, and for monitoring of battery power in pacemakers.
2. Background of the Art
Over the last several decades, the lithium-iodine battery has been widely adopted as a power source for the pacemaker industry, as well as other applications. Indeed, the broad use of this battery system in the pacemaker industry has resulted in it becoming substantially the standard power source for that industry. Well over 1,000,000 such batteries have already been implanted. The lifetime of such cells is not great enough to outlast the patient, so the industry has experienced occurrences where such batteries have come to their End Of Life (EOL) under nominal loads and normal circumstances. In addition to the actual replacement experiences of depleted batteries, methods have been developed for approximating the EOL curve of such batteries. It has become evident that there is a great need for matching or interfacing the device being powered by the battery with the parameters of battery behavior to optimize EOL operation.
In the practice of the invention of this application, reference is made to "lithium systems," meaning lithium-type battery cells. As pointed out in the article of Parsonnet et al., American Heart Journal, October 1977, Vol. 94, No. 4, pp. 517-528, in 1977 there were at least 5 types of lithium systems in widespread use, including lithium iodine types such as made by Wilson Greatbatch, Ltd. and Catalyst Research Corporation. Today an even wider number and variety of lithium batteries available. This invention is directed particularly, but without limitation, to battery systems, especially the lithium battery systems characterized by having an internal resistance characteristic curve which is initially substantially linear as a function of energy depletion, but which asymptotically approaches an energy production (output) limit and increasing internal (e.g., DC) resistance. This total output maximum and high internal resistance is found near, but well before EOL. At that time, the linear characteristic relationship between energy depletion and internal resistance exhibits a knee and internal resistance rises rapidly. This characteristic of lithium-type sources is discussed in my U.S. Pat. No. 4,031,899 which patent is incorporated herein by reference. In the lithium iodine type battery, the cell cathode may consist of molecular iodine weakly bonded to polyvinyl pyridine (P2VP). At beginning of battery life in this type of system, there are about 6 molecules of iodine to each molecule of P2VP. No electrolyte, as such, is included in construction of the cell, but a lithium iodine (LiI) electrolyte layer forms during cell discharge, between the anode and cathode. The LiI layer presents an effective internal resistance to Li.sup.+ ions which travel through it. Since the LiI layer grows with the charge drawn from the battery, or milliamp hours (mAh), this component of the battery resistance increases linearly as a function of mAh (i.e., as a function of cell energy depletion). In the pacemaker environment, since there is constant (but not uniform) energy depletion, this component of the internal resistance increases continually with time. However, and particularly for a demand pacer which at any time may or may not be delivering stimulus pulses, the increase of this component is not linear with time, due to the fact that current drain is not uniformly constant.
For the lithium iodine type cell, there is another component of internal resistance, which is caused by depletion of iodine in the cathode. The cathode is essentially a charge transfer complex of iodine and P2VP, and during discharge of the cell iodine is extracted from this complex. In the beginning there are about 6 molecules of iodine to each molecule of P2VP. During extraction of iodine from the complex, the resistance to this procedure is low until the point is reached where about only 2 molecules of iodine are left for each molecule of P2VP, at which point this resistance rises very sharply. This gives rise to a non-linear internal resistance component which, for the lithium system, is called variously the depletion resistance, the depolarizer resistance, the charge transfer complex resistance, or the pyridine resistance. By whatever name, the combination of the non-linear component with the linear component produces the resistance characteristic with a knee occurring toward EOL, the knee being caused by the reaching of depletion of available charge carriers from the cathode.
The pacer industry is aware of the potential problem of determining EOL. Since the internal resistance of the source rises drastically after the knee, the battery may very little or no useful lifetime left after the knee has been reached and passed. Some pacer manufacturers have predicated their design for determining EOL upon detection of the battery output voltage, which voltage comprises the constant open circuit voltage minus the drop caused by the current drain across the internal resistance. However, this is a highly tenuous and very unsatisfactory premise for determining EOL, due to the fact that actual current drain near EOL cannot be predicted nor directly measured with ease. For any manufacturer's pacer which is implanted and used in combination with a given electrode, there will be a variation in the effective load as seen by the lithium battery, and a resulting variation in the overall current drain. Accordingly, if the EOL design is predicated upon sensing voltage drop to a given level, there can be very little assurance that the level chosen will closely and accurately correspond to the knee of the cell curve.
One lithium cell manufacturer, Catalyst Research Corporation, in a paper presented to the Workshop on Reliability Technology for Cardiac Pacemakers, October, 1977, pointed out that for such batteries, sensing of the battery internal resistance is more reliable than voltage sensing. The position that cell resistance rather than cell voltage is a better warning indicator is based upon the observation that the resistance characteristic has a much less steep EOL curve. Stated differently, at low currents typical for pacers, plots of resistance against time give more warning than plots of voltage against time. If voltage characteristics for different current drains are plotted, the knees are observed to have a fairly wide variation, meaning that the voltage at which the knee might appear is subject to substantial variation as a function not only of the particular battery being used but also the load being drawn by the pacer. On the other hand, plots of resistance indicate that the knee varies over a smaller range of values of internal resistance. Since the current drain may vary by as much as a factor of 5 to 10 due to different electrode loads (which may be varied between similar units by pre-programming), the variation in voltage may be as much as five to ten times as great as the variation of internal resistance. Monitoring the internal resistance provides a direct indication of the state of the battery, whereas monitoring the output voltage gives only a secondary indication, reflecting both the state of the battery and the operating condition of the pacer. This condition, it is anticipated, will be even more emphasized with the development of new, thin, large area batteries and cells which generally have steeper EOL slopes.
U.S. Pat. No. 4,031,899 discloses a circuit which can be programmed to provide an indication of the internal resistance of the lithium battery cell. A switching circuit is used which alternately connects the relatively high current drain output circuitry and the relatively low current drain remainder of the circuit. By adjusting the duty cycle of the switching function, the voltage transferred from the battery to the respective circuits may be programmed to be substantially constant until the resistance reaches the predicted value correspondingly to the knee of the curve, after which there is a programmed drop. Since the oscillator rate is linear as a function of delivered voltage, a programmed drop in frequency can be used to indicate that the battery resistance has reached the level where the knee was expected. This feature provides a decided advantage over previous EOL designs which sensed voltage. The invention disclosed in U.S. Pat. No. 4,259,639 is described as having the added advantage that the exact resistance value at the knee need not be anticipated ahead of time. Even though the depletion resistance component is very predictable, the total value of the internal resistance at the time the knee is reached will be subject to some statistical variation so that EOL cannot be accurately predicted by simply monitoring total battery internal resistance. What is acutely needed in the pacer industry is a means for providing an accurate indication of the EOL and more preferably an accurate identification of an Elective Replacement Time (ERT) when a battery has reached a sufficiently depleted level that replacement procedures should be scheduled at a convenient time in a defined future period. The same need exists in other industries, such as electronic computers and space vehicles where batteries are used to maintain power during shut off of normal power or in emergencies.
U.S. Pat. No. 4,259,639 describes an improved method and circuitry means for detecting the occurrence of the knee of the curve of the resistance characteristic of a battery source having non-linear energy depletion characteristics. The circuit interfaces with a battery and is designed to measure that component of the internal resistance, which causes the knee in the internal resistance characteristics, and to provide an output which is useful for control of the device being driven by the battery. In the preferred embodiment, a high speed switching circuit is provided for measuring short circuit current drain from the battery over a small time duration, so that the short circuit current reflects the depletion component of the resistance which causes the knee, and does not reflect the internal resistance component. The circuit utilizes the fact that the electrical representation of the battery source comprises the linearly increasing resistance component in parallel with an effective capacitance, such that for very short samples of short circuit current from the battery, the linear component is shunted. By comparing the sampled short circuit with a predetermined value corresponding to a given level of the depletion resistance, which level is selected to clearly indicate the knee of the resistance characteristic, there is obtained an unambiguous indication that the battery is near end of life. One problem with this design is the need to essentially close down the electronics of the system, impress a significant voltage through the battery section to effectively override any transient current or signals, and read the data while the system is closed. In systems where continuous performance or minimally interrupted performance of power flow and system performance is required (as in pacemakers, difficult to replace battery systems, and the like), unnecessary shut down of the system in periodic or even infrequent intervals to test for battery depletion involves an element of risk that can not always be justified.